Step Height Measurement by Using Heterodyne Central Fringe Identification Technique W. T. Wu*, H. C. Hsieh, Y. L. Chen, W. Y. Chang, and D. C. Su Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, 1001 Ta-Hsueh Road, Hsinchu 30050, Taiwan, ROC ABSTRACT A simple method for measuring a step-height sample is presented with the heterodyne central fringe identification technique and a precision translation stage. This method can accurately point out the zero optical path difference position such that the optical path lengths of two arms of the interferometer are absolutely equivalent. Thus, the two surfaces of the step-height sample can be identified sequentially with the translation stage. The displacement of the translation stage is equal to the step-height of the test sample. The feasibility of the technique is demonstrated. The measurable range is not limited by the coherence length of the light source. The measurement accuracy depends on the uncertainties of the heterodyne central fringe identification method and the translation stage. In our setup, we have a 100 mm measurable range and a 4 nm uncertainty. The wavelength stability of the light source has a minor effect on the measurement. Keywords: central fringe, heterodyne interferometry, electro-optic modulator, tunable laser, translation stage, step- height. 1. INTRODUCTION The accurate length measurement technique is very basic and important in science researches and industrial products. It will affect the research results and the product qualities. The interferometer with a laser light source can obtain very good results in wavelength unit and it is also a non-destructive technique, so it is often used for the accurate length measurement. On the other hand, the lengths of two arms of the Michelson interferometer [1] with white light source can set to be equal as the maximum intensities of the interference fringes are appeared. That technique is known as the central fringe identification technique [2-7]. The resolution is about 1~2 μm. To enhance the measurement resolution, Lee et al. [8] proposed an improved method with the heterodyne interferometry [9-12]. This method can accurately point out the zero optical path difference position such that the optical path lengths of two arms of the interferometer are absolutely equivalent. In this paper, a simple method for measuring a step-height sample is presented with the wavelength scanning heterodyne central fringe identification technique and a precision translation stage. It can more easily identify the moving direction and phase variation (can exceed 2π) of interference fringes to induce the required moving direction and almost distance of reference mirror for the zero optical path difference position. In the measuring procedure, first, one surface of the step- height sample can be identified when the lengths of two arms are equal. Secondly, the reference mirror is displaced with the translation stage and another surface of the tested sample can be identified similarly. The displacement of the translation stage is equal to the step-height of the test sample. The feasibility of the technique is demonstrated. The measurable range is not limited by the coherence length of the light source. The measurement accuracy depends on the uncertainties of the heterodyne central fringe identification method and the translation stage. In our setup, we have a 100 mm measurable range and a 4 nm uncertainty. The wavelength stability of the light source has a minor effect on the measurement. Hence, it has some merits such as high accuracy, long measuring range, simple optical structure, and easy operation. *wuwonds@gmail.com; phone +886-3-573-1951; fax +886-3-571-6631 Instrumentation, Metrology, and Standards for Nanomanufacturing IV, edited by Michael T. Postek, John A. Allgair, Proc. of SPIE Vol. 7767, 77670J · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.860356 Proc. of SPIE Vol. 7767 77670J-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 04/25/2014 Terms of Use: http://spiedl.org/terms